Getting a Grip on DOCSIS Network Reliability Equals Increased Revenue

Networking ADVANCED NETWORKING TECHNOLOGY

Getting a grip on DOCSIS Network reliability equals increased revenue

By Brady Volpeg, Sigtek tended destination in a timely manner first describe the origin of many common and without errors. DOCSIS network problems. This is best ata-Over-Cable System Interface This model is best illustrated as a bottom- done with a bottom-up approach, discussing Specification (DOCSIS) is the ve- up architecture in Figure 1. The base of the impairments at each layer of the DOCSIS D hicle for cable operators to obtain pyramid represents the physical layer of pyramid and how to best identify the im- immediate revenue-generating opportuni- DOCSIS. This is where data is modulated pairment. Only after the problem has been ties such as broadband data, voice-over- and up-converted to an RF carrier for trans- defined can problem resolution begin. IP, IP video-on-demand, and countless port across the HFC plant. The second lev- emerging IP-based technologies. These el of the pyramid is the DOCSIS Media RF plant impairments growth channels rely on cable operators Access Control (MAC) layer. This Without question, the majori- obtaining and retaining subscribers. represents the inter-communica- ty of DOCSIS network prob- Subscriber satisfaction with new services is tions between DOCSIS-based lems occur at the physical or a direct function of DOCSIS network reli- devices that enable cable RF plant level. This is due ability. Improving DOCSIS network relia- modems (CMs) to commu- to the fact that the RF bility requires new skills and new test equip- nicate with the cable mo- network, particularly ment, in addition to the skills and test equip- dem termination sys- in the upstream, is ment the industry possesses today. tems (CMTSs). inherently rich This article provides a working knowl- Finally, the top with mecha- edge of DOCSIS networks, the types of level of the nisms that impairments that exist in DOCSIS net- pyramid rep- will pre- works, and strategies employed in identi- resents the lay- vent or fying the impairments in the physical plant. er at which impair Ethernet data DOC- DOCSIS working model is transported. SIS Developed by CableLabs, DOCSIS is This layer is used communica- the specification which provides a standard both by subscribers Figure 1: DOCSIS working model. tions. Some very for bridging Ethernet data over a hybrid for data access and by common and fiber/coaxial (HFC) network. The DOCSIS cable modems during registration well-known RF impairments are: specification defines the method by which (explained later). • Linear impairments such as Micro-re- DOCSIS-based devices operate on the RF flections; amplitude and group delay dis- plant. Fundamentally, there are three layers of Troubleshooting DOCSIS tortion; and amplitude slope or tilt. communication which must be understood In assessing and improving DOCSIS • Non-linear impairments such as com- in order to analyze a DOCSIS network: network reliability, it is critical that all three mon path distortion (CPD); and return 1. The physical layer (PHY), which de- layers of the DOCSIS working model are laser clipping. fines how DOCSIS-based devices trans- analyzed. Impairments that occur in the RF • Transient impairments such as ingress mit data on RF carriers on the RF plant, plant may appear to be DOCSIS- or IP-re- noise; and impulse noise. 2. The DOCSIS communication lan- lated problems; similarly, problems in the Individually, these impairments may guage, or protocol, that defines how DOCSIS MAC or IP protocols may appear impact DOCSIS reliability and can be dif- DOCSIS-based devices communicate with as RF impairments. This creates an enor- ficult to identify using conventional test each other, and mous troubleshooting dilemma, which of- equipment. Collectively, these impairments 3. The method that Internet Protocol ten results in significant time and money create an environment where it is impossi- (IP) packets are transported over the spent attempting to resolve a problem ble to identify the source of network failures DOCSIS network, arriving at their in- which does not exist. Here lies the enigma with certainty. Why is this so? behind many DOCSIS network prob- In a DOCSIS network, there is one E-mail: lems–locating the source of the problem! master controller, the CMTS, commu- In order to solve the enigma, we must nicating with hundreds or thousands of

48 CED/NOVEMBER 2004 Networking ADVANCED NETWORKING TECHNOLOGY

transmission. A DOCSIS protocol analyzer can filter on a specific cable modem and provide signal quality measurements, as shown in Figure 2. Armed with this capability, a cable technician can look at the signal performance of a single cable modem experiencing performance issues and determine whether or not the performance issues are due to RF impairments, without ever leaving the head- end or hub site. Figure 2 depicts a cable modem trans- mitting in 16-QAM, with MAC address 00:E0:6F:29:27:AC, which exhibits a 24.0 dB MER. Because 16-QAM transmissions only require 18 dB MER, the technician can be certain that this modem is operating properly in the RF plant. If, on the other hand, the MER of this par- ticular modem was, for example, 16 dB, then the technician would know that the modem is experiencing impairments from the RF plant. The technician now knows whether the source of the network impairment is a problem in the RF plant is a communications related problem, not involving the RF plant. In the case of RF plant impairments, tradi- tional test equipment can be used for impairment location and resolution. For non- RF plant related impairments, the DOCSIS protocol analyzer must be used for analysis of the RF modulated data.

DOCSIS protocol impairments CableLabs and tComLabs are the only two organizations responsible for certifying and qualifying cable modems and cable modem termination systems as DOCSIS Figure 2: Upstream MER of a single cable modem. compliant. This expensive and time- consuming testing process attempts to test cable modems. The cable modems all modems collectively. While these mea- each device against the DOCSIS specifica- transmit signals to the CMTS by time- surements provide a qualitative index tion under a number of possible real-world sharing the upstream channel, also called of overall network performance, they conditions in the laboratory. However, Time Division Multiple Access (TDMA). fail to provide the performance of indi- DOCSIS certification does not always guar- This means that the modems transmit vidual cable modems. antee that all DOCSIS-based devices con- data for a very brief period of time–on Analyzing individual cable modem per- form to the DOCSIS specification. the order of micro-seconds–and turn formance requires a new breed of test A previously-certified cable modem can their transmitters off. equipment called DOCSIS Protocol become non-conforming with a minor Conventional spectrum analyzers and Analyzers. These analyzers demodulate in- product change by either the equipment vector signal analyzers enable the tech- dividual cable modem transmissions and vendor or a manufacturing process change. nician to determine the carrier-to-noise provide not only a quantitative measure- These seemingly minor changes can have (CNR) level and modulation error ratio ment of the signal’s quality, but also provide catastrophic impacts on equipment per- (MER), respectively, of the cable information designating the source of the formance in DOCSIS networks.

50 CED/NOVEMBER 2004 When DOCSIS-based devices violate a Trivial File Transfer Protocol (TFTP) ventive maintenance. This second method, the DOCSIS protocol, network failures server. Both the DHCP and TFTP servers coupled with the proper test equipment, is are sure to follow. Using conventional RF transmit data over the IP network. the key to network reliability. At the phys- test equipment in identifying the source of After cable modem registration, the IP ical layer (RF), most systems have in place DOCSIS protocol violations is difficult, if layer is used by subscribers to transfer and one or more monitoring systems for mea- not impossible. Some diagnostic infor- receive IP data such as e-mail, Web pages, suring ingress levels, CNR, inter-modula- mation is provided by the CMTS, but this and gaming sessions. Additionally, the IP tion distortion levels, etc. This is critical for usually only identifies that a problem ex- layer is where VoIP calls reside. IP layer im- early identification of common problems. ists and does not show the root cause of pairments such as server failures, network DOCSIS protocol analyzers also serve the problem. congestion, data delay and jitter, will pro- as a critical element to achieving network In order to identify and isolate DOCSIS vide a less than satisfactory user experience. reliability. DOCSIS protocol analyzers, protocol violations, one must analyze the The IP layer also suffers from any RF or when not being used for tactical purposes, protocol in its native format–as RF modu- DOCSIS related impairments. For this can monitor downstream and upstream lated carriers in the RF plant. Additionally, reason, the IP layer resides at the top of the data communications of all DOCSIS de- DOCSIS commands have a format unlike DOCSIS working model pyramid. Failures vices. This information is archived to a any other in the communications industry, at any sub-layer directly translate to ap- database, which provides trend analysis as defined by the DOCSIS specification. parent failures at the IP layer. Failures at the information on cable modem MER, pow- Cable operators must again turn to IP layer translate to network reliability er, frequency, timing and equalization ad- DOCSIS protocol analyzers in order to problems and subscriber complaints. justments. User definable thresholds can be troubleshoot DOCSIS protocol-related set which automatically notify appropriate problems–the very same way the DOCSIS Achieving reliability personnel when device operation becomes vendor community develops and trou- Improving DOCSIS network reliability critical due to any number of impairments bleshoots DOCSIS-based CMTSs and CMs. is not a single action or investment. Rather, in the DOCSIS pyramid. The result is a DOCSIS network that it is a conscious decision to implement a cra- This latter method provides a means of is not operating properly. The technician dle-to-grave strategy to work the DOCSIS identifying potential network failures be- has examined the RF plant and deter- pyramid from the bottom to the top. This fore they occur. The ability to replace a fail- mined that the problems are not the result begins with investing in quality transmission ing modem, upstream transmitter, or of RF impairments. Close analysis of equipment and using proven practices for CMTS, long before the subscriber experi- DOCSIS communications between the installing, balancing and maintaining the ences an outage, this translates directly CMTS and the cable modems experienc- HFC plant. Education and training play a into improved network reliability, from ing problems will determine if equipment strategic role in DOCSIS setup and main- the point-of-view of the subscriber. is functioning properly. A DOCSIS pro- tenance, ensuring that the correct equip- Problems will still occur, but they are re- tocol analyzer captures DOCSIS protocol ment is chosen and properly configured. solved before the end user notices them. and a viewer which presents the DOCSIS Finally, investing in the proper test equip- protocol in a format that is readable by the ment, training technicians how to use the Summary human eye. This output is a powerful tool equipment, and actually putting the equip- Network reliability is the single greatest for both identifying the presence of DOC- ment to use, is the final step. instrument to obtaining and retaining pay- SIS violations and for settling disputes There are two principal ways to employ ing customers. It is the metric that will de- with vendors providing equipment that vi- test equipment in order to improve termine the successful deployment of ad- olates the DOCSIS specification. DOCSIS network reliability–tactically and vanced IP services beyond simple e-mail strategically. Tactical network testing is the and Web browsing. No longer is it an option IP layer impairments usual practice, also referred to as “fire or a luxury–network reliability is a necessi- IP layer impairments impact both the fighting.” This occurs when a problem ty for the cable industry to successfully cap- subscriber and the DOCSIS network. It happens, usually a subscriber-generated italize on the video/voice/data triple play. is obvious that for a cable modem to reg- complaint or a major outage. The required As with any new technology, for exam- ister with a CMTS, both the RF plant test equipment is located, the dust is blown ple fiber optics and digital video, the in- and the DOCSIS MAC must be func- off of it, and someone is identified who re- dustry has learned new techniques and tioning properly. But the IP layer is also members where the power button is lo- adopted new technologies in order to make critical for cable modem registration. cated. Testing commences, fingers point, the technology successful. DOCSIS is sim- First, the cable modem must receive a speculations are sounded, and eventually ply a new technology. The tools and train- unique network address from a Dynamic the impairment is identified or the problem ing for implementing and stabilizing Host Configuration Protocol (DHCP) goes away, to surface again someday. DOCSIS networks are available. It is now server. Additionally, the cable modem Strategic network analysis is a very dif- a matter of execution for cable operators to must download a configuration file from ferent approach–also referred to as pre- directly increase profitability.